(corrected from original entry) In the current version, the target track must match the track onto which the multicam clip (a nested sequence) has been edited. Therefore, if the clip is on V2 and target tracks V1 and V2 are enabled, you won't see the cameras in the preview. If you turn off V1, they will appear. It seems like this is a bad design. You should be able to preview independently of the enabled target track. Or at the very least there should be a preference setting to follow the targets or not.
[Oliver Peters]" so it's clearly a Premiere Pro design issue."
Yes, this is a bad design choice. The idea that the lowest targeted track, as opposed to the highest, engages all the multicam features makes no sense whatsoever. Better than that of course would be to default to the highest track without needing to target it, the FCP Legacy method, UNLESS a lower track were targeted. This would give you the most intuitive response while also giving you a way to over ride for special needs.
nothin' attached to nothin'
"Deciding the spine is the process of editing" F. Bieberkopf